How do I know which noun is masculine which is feminine?

In portuguese nouns have gender- masculine or feminine. 

Most masculine verbs end with -o, -im, -om, and -um. Most feminine nouns end with -a, -ção, -dade or -gem.

Examples:

Masculine

o carro = a car

o fim = the end

o atum = tuna

o tom = the tone

Feminine

a pargem = the stop

a verdade = the truth

a noção = athenation

a formiga = the ant